1. Field of the Invention
This invention relates to a method and apparatus for electrochemical analysis of fluid samples, and also to an aqueous solution useful in such method and apparatus. The invention is especially applicable in the medical field, but is not limited thereto. The invention will be described below particularly in relation to the analysis of blood serum and urine samples, but is applicable to the analysis of a wide variety of other samples.
2. Description of the Related Art
In electrochemical analysis, the response time of an electrochemical sensor in which ion transport takes place is often relatively slow. It is generally sought to improve the response time and thus the sensitivity of the sensor. One known form of sensor is the ion selective electrode in which a sample to be measured contacts an ion selective membrane containing ligands which specifically transport the ion being measured. An internal solution of the electrode, which typically contains an electrolyte of the gel type, also contains the ion being measured. An electromotive force (emf) is generated in the cell as a result of the concentration gradient across the ion selective membrane, and this emf is measured in order to measure the concentration of the selected ion in the sample solution. Strictly speaking, it is the ionic activity in the sample solution which is measured, not the concentration, but the two are related.
Such ion selective electrodes are used for example in blood serum analyzers. Typically, the prepared sample of blood serum is passed consecutively through three ion selective electrodes, which measure the concentrations of sodium ions, potassium ions and chloride ions in the sample. Such analyzers are nowadays large machines which perform a large number of other analytical measurements on blood samples. For convenience, for example in a hospital, it is also desired to analyze urine samples in the same machine, in particular to measure the concentration of sodium, potassium and chloride ions in a urine sample using the same ion selective electrodes. The problem arises that while the characteristics of samples derived from blood, e.g. serum or plasma, are generally similar, the characteristics of urine in relation to the ion selective electrode are very different from those of samples derived from blood. Because the ion selective electrode has a relatively long response time, its response to a urine sample can be unacceptably slow.
One solution to this problem is to batch the samples, so that urine samples are presented to the ion selective electrodes separately from samples derived from blood. However, this is inconvenient for the operator of the machine, when presented with a large number of samples which arrive for analysis in a random order, or when a particular sample must be analysed urgently.
A further problem in the prior art can be the frequent need to perform a priming operation, i.e. to fill the whole flow passage of the apparatus with a diluting solution, e.g. to put it into a uniform condition. This takes time and consumes diluting solution. Particularly, if samples are batched according to type, priming after each batch is thought necessary.
One known form of blood analysis machine is the "TBA-50S" of Toshiba. To enable serum samples and urine samples in a random order, this machine is arranged to dilute urine samples preliminarily and for this purpose a special dilution line is added. See "TBA-50S Development of Urine Measuring System", Japanese Journal of Clinical Laboratory Automation, 12(4), page 78, 1987. Such analysis machines dilute all samples considerably for the electrochemical analysis, but the additional preliminary dilution of a urine sample leads to very low values of sodium and chloride ions, which then become difficult to measure. Furthermore, if the preliminary dilution is an additional step performed by the operator, this is inconvenient. If the machine performs the additional dilution, the machine must be larger and more complicated.
Another document describing analysis of serum using an ion selective electrode is JP-A-60-73359 (1985), which describes a process in which the ion selective electrode is contacted first with a solution for calibration, i.e. an internal standard solution, then with the serum sample, and then with a cleaning solution which has characteristics similar to those of the internal standard solution. Measurements are taken for the calibration solution and the sample. Attention is also drawn to U.S. Pat. No. 4,680,270 which describes the use of ion selective electrodes in the analysis of blood samples.